When stacks deteriorate, they can fall over in high wind. The higher the stack, the greater the danger of it falling over once it has deteriorated. Similarly, the higher the stack, the greater the risk of an accident while the stack is being removed.
All stacks at nuclear power plants, and most modern stacks, are made of steel. Over time, they will corrode, particularly at the base. For removal, stacks are cut at the base and then taken down with a crane.
Where the inside of the stack may be contaminated with hazardous materials, it is necessary to contain any debris that might be created during removal. For example, stacks at nuclear facilities may be contaminated with radioactive substances. These stacks can be as high as 100 feet, and therefore have a high risk of falling over while they are being taken down. Since they may contain radioactive debris that can be dislodged during removal or afterwards, it is important to seal the ends as quickly as possible.
The conventional method of removing a stack that may be contaminated is "bagging off." The top of the stack is sealed. Then, a large sleeve is placed over the area to be cut. The sleeve is affixed at its top and bottom, leaving slack. The sleeve has a number of glove ports through which tools can be manipulated to cut the stack. Once the stack has been completely severed, it is lifted with a crane. The sleeve is cut and the slack is used to bag off the cut end of the stack. The stack is guided to a horizontal position on the ground.
This procedure is acceptable for smaller stacks. However, the danger of wind knocking the stack and crane over increases with the size of the stack and the length of time it must be suspended. What is needed is a means for sealing a stack before it is lifted by the crane.